My son is autistic and he is going to school out of district , i just got informed that he is going back to district but we know that is a big mistake

You need to meet with the school immediately.  If the school has changed his placement, this is something they should have discussed with you before making the change.  If he does not already receive compensatory education for the two years that he missed, he might be entitled to it.  The district may have developed an in district autism program but they still must show that your son's goal and objectives in his IEP can be met by this program.   Look at the factors that initially got him the out of district placement.  Was there an evaluation of his educational needs, and a comparison to the out of district school and how that school would meet his needs?  Has the school proposed an in district placement that will meet the same needs?  If not, it is something you may have to litigate with the district.